An intermediate complexity moist General Circulation Model is used to investigate the sensitivity of the Quasi-Biennial Oscillation (QBO) to resolution, diffusion, tropical tropospheric waves, and parameterized gravity waves. Finer horizontal resolution is shown to lead to a shorter period, while finer vertical resolution is shown to lead to a slower period and to an accelerated amplitude in the lowermost stratosphere. More scale-selective diffusion leads to a faster and stronger QBO, while enhancing the sources of tropospheric stationary wave activity leads to a weaker QBO. In terms of parameterized gravity waves, broadening the spectral width of the source function leads to a longer period and a stronger amplitude although the amplitude effect saturates when the half-width exceeds $\sim25$m/s. A stronger gravity wave source stress leads to a faster and stronger QBO, and a higher gravity wave launch level leads to a stronger QBO. All of these sensitivities are shown to result from their impact on the resultant wave-driven momentum torque in the tropical stratosphere. Atmospheric models have struggled to accurately represent the QBO, particularly at moderate resolutions ideal for long climate integrations. In particular, capturing the amplitude and penetration of QBO anomalies into the lower stratosphere (which has been shown to be critical for the tropospheric impacts) has proven a challenge. The results provide a recipe to generate and/or improve the simulation of the QBO in an atmospheric model.

The predictability of stratospheric sudden warming (SSW) events are considered in 10 subseasonal to seasonal (S2S) forecast models for 10 SSWs over the period 1999-2009. The 10 SSWs are divided into those with above-average predictability (in one case exceeding 20 days), below-average predictability, and average predictability. The four factors that most succinctly distinguish the composite with above average predictability are an active Madden-Julian Oscillation with enhanced convection in the West Pacific, the Quasi-Biennial Oscillation phase with easterlies in the lower stratosphere, a strong SSW, and a strong pulse of wave activity in the week before the event. Other factors, such as El Nino, stratospheric preconditioning, and the morphology (split vs. displacement) are comparatively less important.

A sudden stratospheric warming (SSW) happened in September 2019 in the Southern Hemisphere (SH) with winds at 10hPa, 60°S reaching their minimum value on September 18. The evolution, favorable conditions, and predictability for this SSW event are explored. The favorable conditions include easterly equatorial quasi biennial oscillation (QBO) winds at 10hPa, solar minimum, positive Indian Ocean Dipole (IOD) sea surface temperatures (SST), warm SST anomalies in the central Pacific, and a blocking high near the Antarctic Peninsula. The predictive limit to this SSW is around 18 days in some S2S models, and more than 50% of the ensemble members forecast the zonal wind deceleration in reforecasts initialized around 29 August. A vortex slowdown in evident in some initializations from around 22 August, while initializations later than 29 August capture the wave-like pattern in the troposphere. The ensemble spread in the magnitude of the vortex deceleration during the SSW is mainly explained by the ensemble spread in the magnitude of upward propagation of waves, with an underestimated tropospheric wave amplitude leading to a too-weak weakening of the vortex. The September 2019 SH SSW did not show a near-instantaneous downward impact on the tropospheric southern annular mode (SAM) in late September and early October 2019. The Australian drought and hot weather in September possibly associated with the positive IOD might have been exacerbated by the negative SAM in October and later months due to the weak stratospheric polar vortex. However, models tend to forecast a near-instantaneous tropospheric response to the SSW.

Using reanalysis data, observations, and seasonal forecasts, the March Arctic ozone hole events in 1997, 2011, and 2020 and their predictability are compared. All of the three ozone hole events were accompanied by an extremely strong and cold polar vortex. The shape and centroid of the ozone holes are mainly controlled by the simultaneous polar vortex. The March 2020 ozone hole was displaced towards Canada and Greenland, the March 2011 ozone low was evenly distributed over the North Pole, while the 1997 ozone hole was displaceds toward Arctic Russia. The predictability of the 2011 ozone hole event is longer (1–2 months) than the other two (~1 month) possibly due to La Niña and Quasi-Biennial westerly winds, favorable for formation of a strong polar vortex. Surprisingly, an empirical model using a substitute index to forecast the Arctic ozone might be as skillful as the general circulation model with a chemistry module.